Figure 7: Polygonal cells and
glacial flow on the northern
margins of Sputnik Planum.
Credit: NASA/JHUAPL/SwRI
The surface of the plain appears relatively young, being uncratered,
and is broken into a network of polygonal cells, 10 to 40km across, with
their centres rising some tens of metres above their margins, the latter
characterised by darker X- and Y-shaped junctions. Modelling would
indicate that this giant basin is probably filled with a 5 to 10 km-thick
layer of frozen volatiles: nitrogen, methane, and carbon monoxide ices,
but dominated by nitrogen ice. Within this layer solid-state convection
may be occurring, with rising plumes of ice creating the surface polygons, at the edges of which the cooled material sinks back down. This
can be likened to what happens in a pan of soup being gently heated
from below, or a ‘cosmic lava lamp.’ The overturn rate is estimated at
1.5-3cm per year, which would put renewal times of Spunik Planum’s
surface at 500,000 to 1 million years, very young by geological standards. This would explain
the absence of craters on
its surface.
The big questions posed
by these discoveries are
how such activity is driven
on Pluto and what internal heat source(s) could
create convection in this
way? The accretional
heat from the formation
of Pluto would long since
have been lost to space,
as would any residual
heat from early impacts,
such as the one believed
Figure 8: Evidence of glacial flow channels is seen in this oblique view of Sputnik Planum.
to have created the PlutoCredit: NASA/JHUAPL/SwRI
Charon system. And there
is no nearby large body to create tidal heating within Pluto, as is the
case for example, for Jupiter’s moons Io and Europa.
This leaves radiogenic heating – heat released slowly by the decay
of radioactive isotopes within the dwarf planet. While an isotope such
as 26Al, with a short half-life of only 730,000 years, would have been
depleted rapidly in the early days of the solar system, one possible
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